1. Field of the Invention
The present invention relates to an air conditioner in which a plurality of outdoor units each including a plurality of low-pressure shell type compressors are connected, and a method for performing an oil equalizing operation in the air conditioner.
2. Description of the Related Art
An air conditioner, in which a plurality of outdoor units and a plurality of indoor units are connected in parallel to a refrigerant circuit, is well known. Each of the outdoor and indoor units may include, a plurality of compressors. In such an air conditioner, an oil reservoir is provided in each compressor. The oil reservoirs of the compressors are communicated via oil equalizing tubes so that an oil equalizing operation can be performed to prevent occurrence of a phenomenon that oil is not supplied to one or more of the compressors.
An example of such a structure will be described with reference to FIG. 21. In FIG. 21, “A” designates an outdoor unit of an air conditioner. The outdoor unit A is connected in parallel to another outdoor unit B while being connected in parallel to an indoor unit (not shown). The outdoor unit A includes a first compressor 32a and a second compressor 33a connected in parallel. The outdoor unit B includes a first compressor 32b and a second compressor 33b connected in parallel. Refrigerant discharge tubes 39a are connected to the compressors 32a and 33a, respectively. Refrigerant discharge tubes 39b are connected to the compressors 32b and 33b, respectively. The refrigerant discharge tubes 39a and 39b are joined and then connected to the indoor unit. A refrigerant suction tube extends from the indoor unit to the outdoor units. The refrigerant suction tube is branched into refrigerant suction tubes 40a, 41a, 40b, and 41b, which are connected to the compressors 32a, 33a, 32b, and 33b, respectively. Each of the compressors 32a, 33a, 32b, and 33b is a low-pressure shell type compressor, in which the internal pressure of a compressor shell thereof during operation of the compressor is lower than the internal pressure of the compressor shell in a stopped state of the compressor.
The first and second compressors 32a and 33a are connected by an oil equalizing tube 43a to feed surplus oil between the compressors 32a and 33a. The first and second compressors 32b and 33b are connected by an oil equalizing tube 43b to feed surplus oil between the compressors 32b and 33b. The oil equalizing tubes 43a and 43b are connected by a connecting tube 49.
Bypass tubes 59a and 59b are branched from respective discharge tubes 39a and 39b of the compressors 32a, 33a, 32b, and 33b. The bypass tubes 59a and 59b are connected to the suction tubes 40a, 41a, 40b, and 41b, respectively. Check valves 45a and 45b are arranged upstream from respective connections, each of which connects an associated one of the suction tubes 40a, 41a, 40b, and 41b and an associated one of the bypass tube 59a and 59b. 
Bypass opening/closing valves 48a and 48b are arranged at the bypass tubes 59a and 59b, respectively. The oil equalizing tubes 43a and 43b are provided with oil equalizing opening/closing valves 46a and 46b in association with the compressors, respectively.
When an oil equalizing operation is performed during operations of the compressors 32a, 33a, 32b, and 33b of the outdoor units A and B by opening, for example, only the bypass opening/closing valve 48a of the bypass tube 59a connected to the discharge tube 39a of the first compressor 32a of the outdoor unit A, the discharge pressure of the first compressor 32a is applied to the first compressor 32a, so that the oil reservoir of the first compressor 32a has a pressure higher than those of the remaining compressors. Accordingly, when all the oil equalizing opening/closing valves 46a and 46b are opened under this condition, lubricant oil in the first compressor 32a is supplied into the second compressor 33a of the outdoor unit A and the first and second compressors 32b and 33b of the outdoor unit B. On the other hand, when the bypass opening/closing valves 48a and 48b are sequentially opened, lubricant oil is supplied into all compressors 32a, 33a, 32b, and 33b in equal amounts (Korean Patent laid-open Publication No. 2000-337726).
In the above-mentioned conventional air conditioner, however, the pressure of each oil reservoir is hardly increased because the air conditioner has an arrangement in which an increase in the internal pressures of the compressors is achieved by operating the compressors in an opened state of the bypass opening/closing valves 48a and 48b to bypass the discharge pressure of each compressor into the same compressor. For this reason, there is a problem in that the oil equalizing operation must be performed for a prolonged period of time to move lubricant oil. Furthermore, there is a restriction that the length of each of the oil equalizing tubes must be short.
Moreover, it is necessary to install the bypass tubes 59a and 59b and bypass opening/closing valves 48a and 48b in the discharge tubes 39a and 39b of all compressors 32a, 33a, 32b, and 33b in the outdoor units A and B, respectively. It is also necessary to install the check valves 45a and 45b in the suction tubes 40a, 41a, 40b, and 41b of all of the compressors 32a, 33a, 32b, and 33b, respectively. For this reason, the overall arrangement is expensive. Also, there is a problem in that it is difficult to secure a desired reliability because an increased number of constituent elements are used.